Move DoubleBuffer to its own files.

Kernel/DoubleBuffer.h

@@ -0,0 +1,27 @@
+#pragma once
+
+#include <AK/Types.h>
+#include <AK/Vector.h>
+
+class DoubleBuffer {
+public:
+    DoubleBuffer()
+        : m_write_buffer(&m_buffer1)
+        , m_read_buffer(&m_buffer2)
+    {
+    }
+
+    ssize_t write(const byte*, size_t);
+    ssize_t read(byte*, size_t);
+
+    bool is_empty() const { return m_read_buffer_index >= m_read_buffer->size() && m_write_buffer->isEmpty(); }
+
+private:
+    void flip();
+
+    Vector<byte>* m_write_buffer { nullptr };
+    Vector<byte>* m_read_buffer { nullptr };
+    Vector<byte> m_buffer1;
+    Vector<byte> m_buffer2;
+    size_t m_read_buffer_index { 0 };
+};